1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method, and relates to, for example, an image processing apparatus that reads an image at high resolution.
2. Description of the Related Art
Conventionally, in image processing apparatuses such as scanners and copy machines, a signal obtained by a sensor receiving light that is irradiated from a light source onto an original is input to an image processing circuit, and the image processing circuit performs signal processing such as filtering to generate a scanned image.
To perform high-definition image processing in an image processing apparatus such as these scanners and copy machines, uniform MTF (Modulation Transfer Function) characteristics are desirable. However, in an image processing apparatus represented by scanners and the like, the recent movements toward high resolution have made it more difficult to ensure the uniformity of MTF characteristics with high accuracy.
In particular, when mass-producing lenses used in high-resolution image processing apparatuses, there is a tradeoff with respect to accuracy and cost of the lens. For this reason, a certain increase in cost is inevitable in order to develop an image processing apparatus that is provided with MTF characteristics for all pixels. In the assembly process of an optical unit, MTF characteristics can partially deteriorate when fine dust adheres to a part of the optical unit in the light path. Although the assembly is performed in a clean room to prevent the optical unit from being contaminated with dust, advanced technologies and high cost are required in order to achieve a high-level prevention of dust contamination.
In the mature image processing apparatus market, these problems are attracting more and more attention. In other words, there is demand to achieve uniform MTF characteristics at a moderate cost.
To meet such a demand, technology related to a full-color image readout system that measures and corrects chromatic aberration of a readout image has been disclosed (see Japanese Patent Laid-Open No. 2003-189114). The chromatic aberration here is an optical characteristic, and is a physical phenomenon arising due a difference between the refractive index of a lens and the wavelength of light. For example, in a color scanner, when reading out an image through a lens, RGB (red, green, and blue) images need to be read out at different positions on an image pickup device due to chromatic aberration. If there are inconsistencies in assembly of the optical unit (such as the lens) within the image readout system, readout accuracy may suffer from instrumental errors due to displacement in RGB image read positions.
In addition, in image processing apparatuses such as scanners and copy machines, dust or dirt can adhere to a glass face or an orientation plate that reads out an image and cause deterioration in the quality of the image formed from the read image.
In order to prevent such deterioration in image quality, technology that detects the location of a portion of an image that is missing due to dust adherence or the like, and corrects the missing image portion by referring to surrounding normal image portions based on the detection results, has been disclosed (see Japanese Patent Laid-Open No. 2003-101737).
However, in Japanese Patent Laid-Open No. 2003-189114, although optical characteristics are corrected between the respective RGB channels, the MTF characteristics of the optical unit cannot be complemented so as to improve the readout accuracy of an image itself.
Meanwhile, in Japanese Patent Laid-Open No. 2003-101737, dust on a document plate can be detected. However, deterioration and the like in the MTF characteristics caused by dust within the light path, flaws in the optical unit, or the like cannot be detected.